The intensive introduction of advanced composites or composite materials in the primary structures of aircraft has become one of the priorities in the design and manufacture of a new generation of aircraft owing to the possibilities they provide for their structural optimization.
Without aiming to be exhaustive, the advantages of composites can be specified in three fundamental aspects:                Their high specific strength relative to metallic materials which is reflected in an advantageous strength/weight ratio.        Their excellent behaviour under fatigue loading.        The possibilities of structural optimization provided by the anisotropy of the material and the possibility of combining fibres with different orientations, permitting the design of elements with various mechanical properties, adapted to varying requirements in terms of applied loads.        
One of the main advantages from the introduction of composites is the cost saving in assembly operations on account of the high degree of integration of structural elements that it permits. However, this high degree of integration demands adequate quality control.
One of the requirements that arises in the quality control of composite parts used in particular in the aviation industry is the detection of the existence of insulating materials in specific localizations of the parts when these materials are not visible because they are covered by paint or any other non-conductive organic coating.
In the prior art, destructive methods are used for this, such as abrasive methods for removing the organic coating and some of the non-conductive material to permit visual detection of the presence of said insulating materials.
In this specific aspect, as in many others, industry demands non-destructive methods, and the present invention is geared to meeting this demand.
The following terminology will be used in the description of the invention:
Part: Structural element to which the non-destructive method according to the present invention is applied, such as a structural element of an aircraft.
Conductive composite: The basic material used for the manufacture of the part, such as a carbon fibre composite.
Non-conductive material: Material used in the manufacture of the part to provide it with electrically insulating zones in some very precise localizations and of small combined dimension relative to the overall dimension of the whole part. An example of non-conductive material is a glass fibre composite.Organic coating: protective layer applied to the part, after manufacture, such as a layer of paint.